1. The Field of the Invention
The present invention broadly relates to the purification of aqueous media contaminated with organic materials. In one of its more specific variants, the invention is concerned with a novel method of separating aqueous streams contaminated with organic materials into an aqueous fraction relatively concentrated with respect to the organic contaminants, and a water fraction relatively free of organic contaminants. The invention is further concerned with apparatus which is especially useful in practicing the method of the invention.
2. The Prior Art
Industrial waste water streams often contain low concentrations of contaminants and in order to meet the statutory requirements of pollution control laws, must be purified prior to disposal. The contaminants may include organic materials having relatively high molecular weights and low molecular weights having varying solubility ranges. Dissolved and/or precipitated inorganic materials also may be present. Thus, the contaminants may be a highly complex mixture of soluble and insoluble organic and inorganic materials having low and/or high molecular weights and may include contaminant species which are ionic and/or nonionic in character.
When organic contaminants of the above types are present in very low concentrations, such as from 100 to 10,000 parts per million (ppm), when following prior art practices it is often impossible from the practical standpoint, and always expensive to separate the contaminated aqueous stream into (1) a concentrate of the organic contaminants which may be used directly or further processed into useful products, and (2) substantially pure water which may be recycled or disposed of without incurring a pollution problem. A number of prior art processes are technically feasible such as complete evaporation of the contaminated aqueous stream and condensation of the water vapor and recovery of the dry contaminants, or partial concentration followed by crystallization or precipitation of the contaminants and separation of the aqueous phase therefrom. These prior art processes all involve phase changes with prohibitively high energy requirements to effect the concentration of the contaminants. As a practical matter, such processes are not economically feasible due to their high costs and the relatively low value of the contaminant concentrate and/or pure water products that are produced.
A number of membrane purification methods have been developed recently which do not require a phase change such as reverse osmosis (RO), ultrafiltration (UF), and electrodialysis. These membrane purification methods have proved to be very useful in purifying waste water having one or more specific contaminants which meet the requirements of the selected method. However, such membrane methods all have important disadvantages when used alone, or when used in the prior art combinations for the purification of waste water streams of the types described herein.
By way of example, reverse osmosis is practical only when the concentration of the contaminants in the RO concentrate stream is at a sufficiently low level to allow practical operating pressures to be employed such as about 1000 psi and lower. Ultrafiltration as practiced in the prior art is capable of separating only those contaminants which have molecular weights above 10,000 or substances which are precipitated and thus are not in solution. Electrodialysis is effective only when the contaminants are ionic in nature. Thus, no one prior art membrane purification method provides a practical method of purifying an industrial waste water stream which contains contaminants of the types described above.
A number of proposals have been made wherein two or more membrane separation methods are combined for the purpose of overcoming the disadvantages of each method when used alone. However, these prior art proposals are not effective in overcoming the operating difficulties experienced in the purification of industrial waste water streams of the type described herein. Accordingly, the prior art has long sought an entirely satisfactory membrane purification method which is capable of separating an aqueous stream contaminated with small amounts of soluble and insoluble organic and/or inorganic materials of low and/or high molecular weights into an aqueous fraction which is relatively concentrated with respect to the organic contaminants, and a water fraction which is relatively free of the organic contaminants. Preferably, the aqueous organic fraction should be sufficiently concentrated to permit its use directly or its further processing into useful products, and the water fraction should be of sufficient purity for recycle or disposal without a pollution problem. In such event, the two products produced by the purification step would each have utility and the combined value thereof would offset or at least reduce the costs of purification.